Various devices have been utilized over time for the separation of nitrogen and oxygen from air. Many such devices rely on a membrane that is exposed to pressurized air, such that oxygen molecules preferentially (compared to the larger nitrogen molecules) diffuse through the membrane, resulting in an oxygen-enriched gas on one side of the membrane and a nitrogen-rich gas on the other side of the membrane. These gases are also referred to as oxygen-enriched air (OEA) and nitrogen-enriched air (NEA), respectively. The effectiveness of membranes at performing the task of separating gases can be characterized by a trade-off that membranes experience between permeability of the membrane to the gas molecules targeted for diffusion across the membrane versus selectivity of the membrane between the targeted gas molecules and other molecules in the gas mixture.
There are, of course, many uses for OEA or NEA, so there are a variety of applications for devices that separate oxygen and nitrogen, including but not limited to medical oxygen concentrators, atmospheric oxygen supplementation systems, and NEA-based combustion suppression systems. In recent years, commercial and other aircraft have been equipped with fuel tank suppression systems that introduce NEA into a fuel tank headspace or ullage, often by bubbling NEA through the liquid fuel. Such systems require NEA with a nitrogen concentration of at least 90% by volume, and attempt to minimize payload weight and size while maintaining target NEA output across a wide variety of operating conditions.
It is recognized that fuel vapors within fuel tanks become combustible in the presence of oxygen. An inerting system decreases the probability of combustion of flammable fuel vapors in a fuel tank by maintaining a chemically non-reactive or inert gas, such as nitrogen, in the fuel tank vapor space also known as ullage. Three elements are required to initiate and sustain combustion: an ignition source (e.g., heat, electrostatic spark, etc.), fuel, and oxidizer (e.g., oxygen). Combustion may be prevented by reducing any one of these three elements. If the presence of an ignition source cannot be prevented within a fuel tank, then the tank may be made inert by: 1) reducing the oxygen concentration threshold, 2) reducing the fuel concentration of the ullage to below the lower explosive limit (LEL), or 3) increasing the fuel concentration to above the upper explosive limit (UEL). Many systems reduce the risk of combustion by reducing the oxygen concentration by introducing an inert gas such as nitrogen to the ullage, thereby displacing some of the oxygen in the fuel tank with nitrogen.
Membrane devices offer many advantages for use in aircraft applications to provide a source of NEA. However, service life of membrane devices for separating oxygen and nitrogen can be limited by the polymers used in the gas separation module. These polymers are susceptible to damage by air-borne contaminants in the media flow including but not limited to hydrocarbons (HCs), liquid or solid aerosols. Contaminants can affect the service life of the membrane in several ways. Liquid/solid particulates can plug the membrane. Liquids can soak the membrane causing swelling, distortion, and, ultimately structural failure of the affected membrane. Various acids and solvents can damage the inner walls of the composite or layered membranes, causing delamination and membrane deformation. Prolonged exposure to heavy HCs can lead to degradation, cracking, and structural damage of the polymer materials used in forming the membranes.
Contamination of the gas separation membrane can be a problem in any operating environment, but can be particularly problematic for on-board aircraft applications. More specifically, air-borne contaminants may include (but are not limited to): residue products from jet fuel, engine lubricating oil, hydraulic fluid, de-icing fluid, and various ambient pollutants in the atmosphere (exhausts from other aircraft, smog, acid rain, etc.). Especially damaging to the membrane can be contaminants such as ketones, acids, and aldehydes. In general, membranes are susceptible to damage from large hydrocarbon molecules such as those commonly found in the complex combustion products of typical jet fuel-powered aircraft engines.